SHEFFIELD -- They've been at the Crucible Theatre almost as long as the heads on Easter Island -- slowly weathering but so familiar that they never look any different from year to year. These are snooker's biggest fans, fixtures in the ringside seats. They're not actually made of stone, it sometimes just looks like it.

Every year in spring, the finest cueists from across the globe descend on Sheffield for the World Snooker Championship. For these men who can do astonishing things with a pointy stick and a pack of polished balls, the experience can be like torture.

For the hardy souls who dare not miss a single session over 17 gruelling days, often from 10am until late, the event is a cross between a religious pilgrimage and a holiday camp for voyeurs.

"I've been coming 25 years in a row now," says Brian, who is perennially decked out in the colours of his beloved Coventry City Football Club. "It's my three-week annual vacation, my trip to Spain. I can't imagine not being at the Crucible for a World Championship, it's my favourite time of the year.

"The first time I walked through the doors I fell in love with the place. It's such a small theatre but you just can't beat the atmosphere. It's so intimate that even players who have been competing here for years can look terrified, as if they've suddenly forgotten how to hold a cue. It's a true test and the person who leaves with the trophy at the end of it is the worthy winner, every year."

The Crucible Theatre doesn't look like much, squatting as it does between a busy road and a post-War shopping arcade. A few years ago, former world champion Mark Williams described it as a "s***hole". In response, snooker supremo Barry Hearn called it "unique and magical". The truth is somewhere in the middle, for magical things do happen between those concrete walls.

One of snooker's ironies is that while the standard has improved markedly since its 1980s heyday, when it jostled with football for newspaper column inches, the sport has struggled to remain relevant in the 21st Century.

Thirty years ago, snooker players were some of the biggest names in British sport. But the world started moving more quickly, and suddenly these men dressed in waistcoats and dicky bows, either plonked in a chair or stalking a table for the entertainment of a hushed audience, appeared from another age.

But snooker never went under, it just lost its way. In 2010, Hearn took over a creaking ship and he has done a sterling repair job, growing the game outside of the UK, reinvigorating the circuit and helping make its elite players rich.

This year the World Championship celebrates its 40th year at the Crucible, and after rumblings that the event might move to China, it again looks immovable.

For one fan, the tournament's immovability is a blessing and a curse. David has been coming since 1992, spending all 17 days and almost every session of every year glued to the very same seat. That's one hell of a commitment, especially given that David travels all the way from Australia. A World Championship in Beijing would no doubt save him thousands of dollars.

"An obsession? Calm down. But it has become a bit of a habit," says David, who is entitled to his privileged front row seat because of some quirk of an outmoded season ticket system.

"I haven't got a wife or children and while I try not to think about how much it costs me every year, it's not as if I'm borrowing money to come here. Then again, I can't afford to do much else. But I doubt I've got the ticket forever, I imagine I'll die at some point."

For David and his ilk, the World Snooker Championship is a celebration of the cerebral, proof that those who sit patiently in silence can be rewarded with great drama. For while the game is faster and looser than it was, it's still not pool.

In this year's World Championship qualifying tournament, Fergal O'Brien and Dave Gilbert set a record for the longest ever frame, which lasted a little over two hours. O'Brien flopped over the line first, winning 10 frames to nine.

Ten-year-old Harvey, sitting out front with dad John during a mid-session interval, knows exactly how long that longest ever frame lasted -- "two hours, three minutes and 41 seconds" -- which is one of the reasons his mates think snooker is "an old man's game".

"They say to me: 'Why do you watch that rubbish?'" adds Harvey. "But snooker is more exciting than football or rugby."

Remarkably, Harvey witnessed a rare 147 maximum break in his first ever session, by Stephen Hendry against Stuart Bingham in 2012. But Terry Griffiths, Crucible legend and snooker sage, is better placed to gauge what makes the sport so torturous to play and so beguiling for spectators.

"Snooker is a series of disasters followed by a minor miracle," Griffiths told me a few years ago. "The Crucible can be a very lonely place, despite being in the company of so many. In fact, it's the company of so many that's the problem."

Even those crammed into the compact little theatre admit to their fair share of suffering. "For the first couple of days of every World Championship you have to recalibrate, get used to just sitting there for a couple of hours at a time," says Brian, who met his fiancée at the Crucible nine years ago and proposed to her during the 2012 tournament, live on the BBC. Mercifully, Lisa said yes.

Stuart Bingham and his family pose with the World Championship trophy in 2015. Photo by Gareth Copley/Getty Images

"There are plenty of people whose eyelids are drooping and who are pinching themselves to stay awake," adds Brian. "And your bum gets quite numb sitting there for that long." Brian sounds like a 16th Century Puritan. Presumably, if he ever gets too comfortable, he sticks forks in his legs between sessions.

The first afternoon of the 2017 tournament saw Ronnie O'Sullivan in action for the first time. A five-time world champion, the Englishman is arguably the greatest to play the game and the Crucible Bar and bookmakers outside the main theatre are empty for the duration of his opening session.

Nobody dares miss a second of O'Sullivan, a man who elevates snooker into an artform and makes century breaks for fun.

But lurking by the stage door throughout O'Sullivan's turn are a few ticketless souls clutching pens and programmes. These autograph hunters, who camp out in all weather, are a furtive bunch. When I ask a couple of them for a chat about snooker, I receive wordless rebuffs.

When I ask how long they've been coming to the Crucible, they stare into the distance, just over my shoulder.

By the main entrance, another autograph hunter eyes me with suspicion when I request an interview. "I haven't got time," he finally says. "I might miss someone."

With that, he shuffles off and collars the passing Martin Gould, the world No. 18. Having scribbled his signature, the bespectacled quarry, dressed from head to toe in a blue tracksuit and looking not much different to his hunter, retreats to a doorway and sparks up a cigarette.

It is a vignette that helps illustrate one of the essential attractions of the game -- while its elite can earn a fair few quid, most of them remain crashingly down to earth and very much men of the people who pay to see them play.

O'Sullivan emerges from his opening session 5-1 ahead, and John and Harvey emerge from the theatre shortly after. Father and son look suitably impressed.

"I've been visiting for 20 years now and every time I come I get that same buzz," says John. "It's just the intensity, you can hear a pin drop. I'm paranoid about missing a session, in case something special happens. But I've got to take a break every now and again, otherwise I wouldn't be able to cope."

I am reminded of the tale of another snooker fanatic, who was once so desperate for the toilet that he left the theatre between frames and missed a 147 clearance. Scarred by the experience, he now drinks nothing all day. Like one of those heads on Easter Island, it's best just to sit and stare.